In the oil and gas industry, gas from a flowline has long been used to drive a pump to inject a liquid into the flowline. Control of such liquid injectors has been accomplished by regulating the volume of gas delivered to the power end of the pump. Control difficulties are encountered if liquid and solids entrained in the gas are passed through the gas flow regulating valve.
There are many applications in which a very small quantity of liquid, e.g., a pint a day, is to be injected into a flowline. Examples of such applications are paraffin producing wells which can be produced without excessive paraffin deposit if a small but continuous injection of a liquid solvent can be maintained; also similar quantities of corrosion inhibitors and demulsifiers are injected into oil wells. Another large application is the introduction of mercaptans (odorant) into a gas flowline. Generally at the locations of such applications, no electricity is available and the only power fluid available is the fluid in the flowline to be treated. In some applications, it is desirable too inject a chemical in gaseous form into a flowline or process vessel.
Metering pumps for injecting small amounts of a liquid into a flowline achieve low flow rates by reducing the size of the pumping end of the injector so that only a small amount of liquid is delivered with each stroke. Considerable difficulty has been encountered with the check valves failing to close completely and the packing being sufficiently resilient to accommodate the stroke volume to thereby cause the injector to cycle without delivering any liquid. Also difficulties have been encountered in injecting liquids containing solids. Such solids prevent the check valves from completely closing and thus prevent delivery of a relatively continuous supply of liquid.
Combined with the problem of delivering chemical from the injector is an even more serious problem of maintaining a preset flow rate of working fluid into the power end of the injector. To obtain low chemical injection rates (e.g., 1 pt./day) the metering valve for the working fluid must be constricted to such an extent that flow blockage will occur from contaminants, also the working fluid flow rate will change drastically from day to night due to temperature changes and corresponding changes in working fluid viscosity; this changes the chemical injection rate.
Utilizing fluid pressure from a flowline to power an injector to pump into the flowline is shown in U.S. Pat. Nos. 3,410,305; 3,065,679; 2,563,211; and 2,413,029. U.S. Pat. No. 3,998,239 discloses an apparatus for dosing a gas into a flowline using pressure in the line to charge the dosing compartment. None of such prior art devices have been suitable for accurately controlled, relatively low flowrate of injection of a liquid into a flowline.
Injection of a chemical in gaseous form in exact amounts and particularly at low flow rates has not been done acceptably in the past.